Drying of smokeless powder



May 5,1942. w. A. DEW ETAL -2,281,560

DRYING OF sMokELEss POWDER Filed April :l, 1959 MllezrAri/zaffiawATTORNEY Patented May 5, 1942 UNITE- STATES PATT OFFICE DRYING orSMOKELESS POWDER Walter Arthur Dew and Bill Harry Mackey, Wu

mington, Del., assignors'to E. I. du Pont de Nemours & Company,Wilmington, Del., a corporation of Delaware Application April 21,1939,Serial No. 269,104

6 Claims.

added to the other ingredients prior to said mixing operation. Thecolloided mass resulting is formed into grains of the desired size,usually by extrusion in the form of strings of desired crosssection, andcutting said strings to the proper length. The green powder is thensubjected to a solvent recovery operation during which most of thesolvent is removed, the recovery of which is effected by variousmethods. Usually, after the solvent recovery operation, the powder isdried further in order to reduce the residual solvent content therein tothe value deemed necessary for suitable ballistic performance. Incertain types of powders, this is done by leaching the powder in waterat elevated temperatures, the water serving to displace most of thesolvent in the powder. The water-wet powder is then dried by means ofair until the absorbed moisture content has been reduced to a valuewhich is ap- I proximately in equilibrium with normal atmosphericconditions.

During the water leaching process for reducing the solvent content, thepowder grains develop a more or less opaque appearance. It is commonknowledge to those skilled in the art, that, if the opaqueness of thepowder exceeds a certain limit, the powder will not regain the desiredtranslucent appearance during the final air drying operation. Since thedegree of opaqueness which the powder attains during the water leachingprocess varies directly with the solvent present in the powder, theprocessing of the smokeless powders heretofore has involved eithercontinuing the solvent recovery operation beyond the point where it iseconomical to recover the solvent, or subjecting the powder to apreliminary drying prior to the water leaching treatment, in order thatthe solvent content be reduced below the critical limit, said limitvarying with changes in composition and granulation of the powder,

The object of the present invention is an improduction of a.satisfactory smokeless powder when the residual solvent contentisdecreased by water leaching without necessitating preliminary dryingprior to said leaching treatment. A further object is a method of dryingpowder after it has been leached in water, which yields a satisfactorypowder even though the solvent content of said powder exceeds the limitsheretofore considered critical. Other objects will become apparent asthe invention is described more fully hereinafter.

We have found that the foregoing objects are attained by drying thepowder in such -manner that a gradual removal of the moisture iseffected at high temperatures. Such removal of moisture quitesurprisingly permits the formation of translucent grains of powder eventhough the solvent content of the powder subjected to the leachingtreatment exceeds the value heretofore deemed critical. This desirableresult is augmented appreciably if, after the moisture content of thepowder has been reduced to the desired value, the heating of the powderis continued for a predetermined period in a closed system, so that themoisture content is kept constant. This continued heating of the powder,while maintaining a constant moisture content, not only tends to improvethe appearance of the grains but likewise prevents finished powder ofsatisfactory appearance from becoming opaque during long storageperiods.

A comprehensive study of powders subjected to water leaching hasrevealed that the moisture content of the powder thus treated'isinfluenced by the amount of solvent present in the powder at the time itis placed in the water; that is to say, an increase in solvent contentof the powder causes it to absorb more water during the leach-q ingprocess. however, does not prevent the final production of a translucentgrain, even though the grains become more opaque during the leachingtreatment than powders of lower solvent content. That a gradualremoval'of moisture at high temperatures during the final drying ofthe-powder should achieve such results is very unexpected, since.thoseskilled in the smokeless powder art have believed that powder grainsbecame opaque because the water, in displacing the Water-solu- -b1esolvent, e. g; alcohol, caused some very small amounts of nitrocelluloseheld in solution with said solvent to be precipitated, thereby causingthe grains to become opaque. Moreover, it was felt that, onceprecipitation of the nitrocellulose occurred, it could never beeliminated, because This increased moisture content,

the solvent content of the usual powder grain when removed from thewater is too low to dissolve and redistribute said precipitatednitrocellulose, That this theory is not absolutely correct will becomeapparent as the invention is described hereinafter.

The powder to be treated in accordance with our process is placed in asuitable receptacle, and air heated to the operating temperature isforced through the powder by means of a blower or fan, and then returnedto the suction of the blower to be recirculated. This procedure iscontinued until the entire powder bed has been raised to a suitableoperating temperature. A small portion of the moisture-laden air is thendischarged and an equivalent volume of outside air is introduced intothe system. The rate of drying thus .may be readily controlled byadjusting the proportion of outside or new air to be recirculated, thedrying being continued until the desired moisture content has beenattained.

Apparatus adapted for carrying out the invention is illustrated in theaccompanying drawing, which shows a schematic plan of the variousequipment involved. The numeral I represents a receptacle adapted toreceive the powder to be treated, said receptacle having near the top abafiie or other means (not shown) to distribute the drying mediumf Thefalse bottom 2 serves to retain the powder, yet permits flow of gasestherethrough. After the powder has been charged into the container I,circulation of the air in the direction indicated is effected by theblower, the dampers 6 being shut so that the system is closed. Thedesired operating temperature is attained by means of the heater 5. Anyliquid moisture exiting from the base of the powder bed is drainedthrough the line 3, which is provided with a trap or other suitablemeans for this purpose. The damper 6a is disposed in the exhaust pipe Ito permit the same to be completely closed.

Qurpreferred procedure contemplates heating the air to the maximumpermissible temperature and passing said air down through the powder.This downward flow is advantageous because it removes efficiently anymoisture which may be entrained therein. The vapor-laden air exitingfrom the base of the powder bed is recirculated until the grains at thebottom part of the bed have attained the desired temperature; then, the

system is gradually purged by exhausting the moist air from the systemand introducing thereinto air from the atmosphere, the rate of purgingof the system being established by a heat balance involving theparticular operating conditions employed. This purging operation iscontinued until the powder has been reduced to the requisite moisturecontent. Then, the exhaust and intake vents are closed, and heating ofthe powder is continued for a period sufiicient to produce a translucentpowder which will not become opaque during subsequent storage.

The details of operation are illustrated in the following examples.

Example 1 The power employed in this test consisted of a 1200-lb. chargeadapted for use in a 155-mm. gun (.055" web). Following the solventrecovery operation, this powder had received the water leaching processbut had not been accorded the preliminary air dry treatment. After thepowder had been charged into the container, the system was closed, theheater turned on, and air was powder grains.

. being continued for 25 hours.

.tained constant. At the end of two hours, the

temperature at the bottom of the powder bed was 43.5" C. At this timethe dampers were.

adjusted to discharge cu. ft. per minute of moist air and pull in anequivalent amount of outside air. This intake of new air was such thatthe temperature of the air leaving the powder did not change noticeablyfor 4 hours, then gradually rose to about 50 C. after a total elapsedtime of 19 hours. At this point the vents were closed, and circulationcontinued for an additional 26 hours, following which the powder wascooled rapidly by blowing cold air through the bed.

A sample taken at the end of 19 hours was translucent but became opaqueon standing approximately one week. However, a sample taken at thecompletion of the foregoing test remained clear and was at least assatisfactory in appearance as this type of powder is when it has beenaccorded the preliminary air drying treatment, then leached and dried inthe usual manner.

Example 2 Three thousand lbs. of -mm. (.055" web) powder were taken,following the solvent recovery operation, and subjected to a preliminarydrying and then leached in water. This charge was then introduced intothe drying chamber, and air heated to 55 C. was passed into the powderfor 2 hours, the air vents being closed. At this point, the powder atthe bottom of the bed had attained a temperature of 42.0 C. The ventswere then opened and 100 cu. ft. per minute of moist air weredischarged, this procedure During the purging of the system, thetemperature of the powder at the bottom of the bed rose to 48.5" C. Thevents were again closedand circulation continued for an additional 48hours, following which the powder was cooled by blowing cold airtheretageous because said operation is costly as well as time consuming.Likewise, it is no longernecessary to extend the solvent recoveryoperation to the same length as deemed necessary heretofore, because theincrease in permissible solvent content means that the solvent contentof the powder to be leached can be reduced without impairing thephysical appearance of the Moreover, the powder grains produced arecharacterized by substantially uniform moisture content which, ofcourse, is very desirable, because it tends to obviate variations in theballistic performance of the powder.

It will be obvious to those skilled in the smokeless powder art thatmany variations may be' made without departing from the spirit and.

scope of our invention. Thus, the apparatus for carrying out our processmay include, as an embodiment, a dry house of-the type in current use,said dry house being modified in order that it be adapted for carryingout the invention. Consequently we intend to be limited only inaccordance with the following claims.

We claim:

1. A method of drying smokeless powder, which comprises heating a chargeof said powder to a predetermined temperature by recirculating a hotdrying medium through said charge, retaining substantially all of thevapors evolved during said heating period, exhausting the vapor-ladenmedium being recirculated and introducing an equivalent volume ofoutside air while maintaining the powder charge at substantiallyconstant temperature, and then continuing recirculation of drying mediumfor a predetermined period.

2. A method of drying smokeless powder, which comprises placing a powdercharge in a closed system, heating said charge to a predeterminedtemperature by heating the air in said system and recirculating said airthrough the powder, retaining substantially'all of the vapors evolvedfrom said powder during the period required to attain said predeterminedtemperature, exhausting slowly the vapor-laden air at the same'rate atwhich an equivalent volume of outside air is drawn into the system whilemaintaining the powder charge at substantially said predeterminedtemperature, andthen continuing recirculation of the air for apredetermined period.

3. A method of removing water from smokeless powder, which comprisesrecirculating the hot drying medium in a closed system through a bed ofpowder until said powder attains a desired temperature, retainingsubstantially all the vapors evolved from the powder during saidrecirculation, gradually purging the system by exhausting thevapor-laden drying medium to the atmosphere and introducing anequivalent volume of outside air into said system while maintaining thepowder bed at substantially constant temperature, then recirculating thedrying medium for a predetermined period at said temperature, and thencooling the powder to substantially' atmospheric temperature by blowingcold' eous medium in a closed system through a bed of powder until saidpowder attains a desired temperature, retaining substantially all of thevapors evolved from the powder during said recirculation, graduallyremoving the vapors evolved by exhausting the vapor-laden'drying mediumto the atmosphere and introducing an equivalent volume of outside airwhile maintaining the powder bed at substantially constant temperature,continuing the removal of thempor-laden drying medium until said powderattains the desired moisture content, and then recirculating the dryingmedium for a predetermined period.

5. A method of finishing smokeless powder, which comprises steeping saidsmokeless powder in water at an elevated temperature, separating saidpowder from the water, and heating said powder to a predeterminedtemperature by recirdetermined temperature immediately prior to itscontact with the powder bed, retaining substantially all the vaporsevolved from said powder until the powder bed has attained apredetermined temperature, exhausting the vapor-laden medium beingrecirculated to the atmosphere powder, which comprises recirculating ahot gas- 1 and introducing an equivalent volume of outside 'air whilemaintaining said powder bed at substantially constant temperature, andthen recirculating the drying medium for a predetermined period.

' WALTER ARTHUR DEW.

BILL HARRY MACKEY.

